In the prior art, magnetic properties required for the magnetic pole pieces of thin film inductive magnetic heads are high permeability, (See IEEE Transactions on Magnetics MAG-5, 442, E. P. Valstyn and D. W. Kosy, 1969); and high saturation magnetizaton, (See IEEE Transactions on Magnetics, MAG-7, 146, J. P. Lazzari and I. Melnick, 1971). High permeability is required to couple magnetic flux between the coil and magnetic recording medium efficiently, and high saturation magnetization is required to prevent saturation of the pole pieces during the write process. The material compositions used are mostly nickel-iron binary and ternary alloys, where the third constituent, such as chromium, in the ternary alloys increases permeability; or a third constituent, such as rhodium is used to increase corrosion resistance, (See J. C. Suits, "NiFeRh Alloys", U.S. Pat. No. 4,023,965). Silicon-iron alloys are also described in the prior art, as in U.S. Pat. No. 4,049,522.
High permeability is generally achieved using material compositions yielding low magnetostriction. For bulk specimens or thin films with isotropic stress in the plane of the film, permeability is generally maximized at compositions of nickel-iron and silicon-iron alloys which yield small positive magnetostriction. For example, commercial alloys using the trade name, Permalloy, are all iron-rich, thereby yielding positive magnetostriction. Saturation magnetization is also maximized using iron-rich compositions yielding positive magnetostriction. Therefore, the teaching of Permalloy or nickel-iron alloys without specifying composition and the advantages of high permeability and high saturation magnetization implies positive magnetostriction.
Composition ranges have been described which yield positive magnetostriction for part of the range and negative magnetostriction for the other part, as in the aforementioned Suits patent U.S. Pat. No. 4,023,965. However, no preference is taught for the composition range yielding negative magnetostriction. For example, Suits (U.S. Pat. No. 4,023,965) teaches the range 65 to 90 atomic percent nickel, which yields negative magnetostriction. Also, it is known that when rhodium is used in a ternary composition, more than one atomic percent rhodium shifts the magnetostriction toward the positive range.
Valstyn et al, cited supra, have taught the preference for magnetic swithing in the pole tips by the rotation process as preferable to switching by domain wall motion. The advantages of switching by rotation include higher frequency response and higher efficiency, due to higher permeability. Lazzari et al taught the use of multilayered films (alternating magnetic and non-magnetic layers), to promote switching by rotation, and elimination of random pulses in the head output during readback of a recorded pattern. Hempstead et al., U.S. Pat. No. 4,103,315 teaches the use of antiferromagnetic-ferromagnetic exchange bias films to control the magnetic domain structure in thin film magnetic transducers. Nowhere does the prior art teach the use of magnetic pole pieces with negative magnetostriction to promote switching by rotation rather than domain wall motion.
Negative magnetostriction and an anisotropic stress are employed in prior art conventional magnetic recording heads to produce high permeability, for high head efficiency. The magnetic instability that would be found in thin film heads does not occur in conventional heads because the pole pieces in conventional heads are large compared to the size of magnetic domains.
Presently known magnetic head pole pieces used in thin film mangetic heads are made from a nickel-iron alloy. However, such head pole pieces are characterized by positive magnetostriction and are known to suffer from magnetic instabilities which lead to waveform distortion of the readback signal, on-track bit shift, and degraded off-track performance.
In U.S. Pat. No. 3,549,428, issued to J. M. Lommel, a process is described for forming a nickel-iron alloy film having negative magnetostriction on a substrate, and for diffusing copper into the film by annealing in a magnetic field environment to saturate the film, thereby imparting a zero magnetostriction to the alloy film. The objective of Lommel is to arrive at zero magnetostriction in order to realize an increased coercive force in copper-diffused nickel-iron alloy. However, the problem of magnetic instabilities which have been observed in materials having positive magnetostriction were not addressed nor solved.